1. Field of the Invention
The present invention relates to a method of evaluating quality of a laser beam in a laser processing machine, and in particular to a method of evaluating quality of a laser beam output from a laser processing head of a laser processing machine.
2. Description of the Prior Art
In certain laser processing of materials, it is found that the quality of the laser beam is an important factor in producing high quality products. In particular, when using high-power CO.sub.2 lasers for cutting relatively thin sheets of steels, peak-to-peak roughness of the cut kerf may vary from less than a micro-meter to well over ten micro-meters, depending upon the quality of the laser beam being used.
Thus, it is important to accurately and quantitatively evaluate the quality of the laser beam.
Based upon result of the evaluation of the quality of the laser beam, accurately performed are adjustments of various lasing conditions in a laser generator of a laser processing machine, such as alignment of mirrors provided in the generator and condition in supplying of laser gas thereto and the like.
Conventionally, the laser beam quality such as laser beam profile is detected by burning holes in such material as metal foils, or acrylic plastic or card.
However, such method which measure the location of a single isotherm in the material being treated is inaccurate since the location of the isotherm depends upon unrelated variable such as laser power, exposure time, material thickness, surface reflectivity, and the like.
Thus, in order to accurately and quantitatively measure the laser beam quality such as laser beam profile, the present inventor has disclosed, in "Optics and Laser Technology", vol 14 (June 1982) pp. 149-153, a dynamical laser beam analyzer 10 as shown in FIG. 1.
In FIG. 1, a rod 11 is attached to the surface of a circular plate 13 with one end thereof pointing toward the center of the plate 13 and the other end protruding from the edge of the plate 13.
The rod 11 and the plate 13 is rotated by a synchronous motor 15. The synchronous motor 15 is positioned so that when it is operated, the rod 11 is rotated to cross perpendicularly laser beam L focussed by a focussing lens 17.
The laser beam L reflected by the rod 11 is detected, with a high resolution and a high response speed, by a detector 19 placed in a position where the plane containing the laser beam L and the detector 19 is at right angle to the rotation plane of the plate 13. Specifically, the detector 19 is provided with a pyroelectric crystal 19a for converting the optical energy into the electric energy and a loading resistor (not shown) connected to the pyroelectric crystal 19a.
Incidentally, behind the rod 11, a beam sink 21 is provided for absorbing beam energy of the laser beam L transmitted forward, passing by the rod 11.
With the above analyzer, a cross sectional profile of the laser beam L is output from the detector 19 as voltage signal varying in time. Specifically, an output trace shown in FIG. 2 which shows a intensity profile of the laser beam L is obtained with a high resolution and a high response speed.